1. Field of the Invention
The present invention relates to an image forming apparatus that has a positively-charged single layer type electrophotographic photoreceptor and a contact charging member.
2. Description of the Related Art
In consideration of the environment, most of the electrophotographic image forming apparatuses of recent years use the scorotron charging system (non-contact system) that forms a large amount of ozone and a contact charging system that performs roller charging using, for example, a rubber roller. The roller charging where electricity is discharged in a small gap between a photoreceptor and a roller realizes a reduction of the ozone.
Inorganic photoreceptors and organic photoreceptors are used as electrophotographic photoreceptors of the electrophotographic image forming apparatuses. Compared to the inorganic photoreceptors, the organic photoreceptors can be produced easily and have a high degree of freedom in the structural design thereof due to a wide selection of organic materials constituting photosensitive layers.
Examples of the organic photoreceptors include a multilayered photoreceptor, which is obtained by laminating a charge generating layer containing a charge generating agent and a charge transport layer containing a charge transport agent, and a single layer type photoreceptor, which has a photosensitive layer containing the charge generating agent and the charge transport agent together. Especially the single layer type photoreceptor is designed to last long, because its film thickness can be increased by a carrier generated near its surface. In addition, compared to the multilayered photoreceptor, the single layer type photoreceptor can be produced more easily at a lower cost with a single layer coating process.
For this reason, a combination of such single layer type photoreceptor and the roller charging is considered to be able to accomplish an environmentally responsive electrophotographic design.
However, a problem specific to a charging roller is the occurrence of uneven charging, which takes place when electricity is discharged locally from an uneven surface of the charging roller. The uneven charging tends to occur in a positively-charged single layer type photoreceptor. Although the specific reason for the uneven charging is unknown, a possible reason thereof is that the discharged voltages vary according to the materials contained in the same resin layer of the single layer type photoreceptor, which are a charge generating material, charge transport material, and binder resin, thereby causing the local discharge.
Although there is a technique for preventing the local discharge, which is a cause of the uneven charging, by increasing the discharge voltage so that the electricity can be discharged from a section that is unlikely to discharge electricity, increasing the discharge voltage (=influx current to the photoreceptor) facilitates peeling of the film of the photoreceptor, thus reducing the life-span of the photoreceptor. For this reason, a technique for preventing the uneven charging without increasing the charged voltage is required.
In addition, unlike the multilayered photoreceptor in which the charge generating layer and the charge transport layer are separated from each other, the single layer type photoreceptor has a composition of a resin material, which is the main ingredient of a photoconductive layer, and a number of materials such as the charge generating material and charge transport material. According to such a configuration, a photocarrier is considered to pass through these various materials in the single layer type photoreceptor and trap these materials (FIG. 1). This carrier trapping changes the characteristics of the photoreceptor and reduces the charging ability of the photoreceptor (indicated by the charge potential of a photoreceptor drum, which is obtained when a constant current is applied thereto).
When the charging ability of the photoreceptor becomes low, the surface potential of the photoreceptor becomes low as well. The electrification current needs to be increased in order to obtain a predetermined surface potential. However, increasing the electrification current means increasing the amount of electric discharge, which causes a negative effect where the film of the photosensitive layer peels off.
The present invention was contrived in view of these circumstances, and an object thereof is to provide an environmentally responsive image forming apparatus having a single layer type photoreceptor and charging roller, the image forming apparatus being capable of preventing the carrier trapping, film peeling and uneven charging that occur on a photosensitive layer.